The present invention relates to a marine riser with a hinged foot suitable for offshore hydrocarbon production.
The prior art marine risers which are conventionally used to connect an underwater oil installation (such as a wellhead, an underwater manifold, etc) to a production platform located at the water surface, comprise a central pipe equipped with a plurality of satellite pipes. A first group of these satellite pipes consists of the production pipes through which the hydrocarbons flow upwardly to the production platform. Through the central pipe of the riser, the hydrocarbons can be reinjected from the platform into one or several oil or gas conveying pipes. A second group of satellite pipes of the marine riser is made up of water injection pipes, or/and of pipes for injecting gas under pressure from the platform, so as to maintain a sufficient pressure in the hydrocarbon deposit.
A third series of satellite pipes of the riser column is made up by the servicing pipes which permit application of a lifting pressure to instruments or tools which are pumped in a counter-current direction relative to the production fluid through the pipes of said first series of satellite pipes, according to the Through Flow Line (T.F.L.) method.
Connection of such a riser column to the water bottom raises difficult problems.
British Patent Specification No. 1,285,530 describes a riser column having satellite pipes helically wound around a central pipe and secured thereto. Such a riser column does not allow important variations in the position of the floating installation and thus, is not suitable for use at water depths which may reach or even exceed 300 meters.
For this application there have been proposed riser columns equipped with a flexible assembly, comprising for example flexible coupling or "flex-joints" which permit substantial displacement of the floating installation.
In this type of installation it is however, necessary to provide means for preventing the satellite pipes from being subjected to excessive stresses likely to break them, as a result of the riser bending.
To solve this problem U.S. Pat. No. 3,701,551 proposes deforming each satellite pipe at the level of articulation of the riser column to form a loop which is substantially contained within a vertical plane.
This solution is not applicable to the riser columns which are contemplated at present, since such columns will include about twenty or even more satellite pipes.
As a matter of fact, the bulkiness of the so-formed loop would be prohibitive. Moreover, the stresses developed in these loops would still be excessive for the loops which are not contained in the bending plane or the riser column.
Another solution has been proposed in FRENCH Pat. No. 2,377,570, consisting in the provision of universal or Cardan coupling on the riser column, and in the deformation of each satellite pipe along two spirals coaxial to the two articulation axes.
As in the above solution, this embodiment is not suitable for a riser column comprising a high number of satellite pipes.
Moreover it is difficult to ensure continuity of the central pipe of the riser column when two sections of this pipe are hingedly connected to each other by means of a universal coupling.
It is known to helically wind the satellite pipes around the central pipe of a riser column so as to permit substantial displacement of the free lower end of these satellite pipes, and to facilitate their connection to corresponding pipes secured to the wellhead (FRENCH Pat. No. 2,399,609 and U.S. Pat. No. 3,189,098), or to permit relative sliding movement of two consecutive sections of the central pipe, with a view toward compensating for vertical movements of the floating installation (U.S. Pat. No. 3,718,183). In all cases the helical portions of the satellite pipes are extended by bent tube sections which are connected to the satellite pipes and are secured to the central pipe. It would thus seem possible to adapt such a technique to a riser column whose central pipe is provided with a flexible coupling or flex-joint. This solution is described in FRENCH Pat. No. 2,376,987 and in U.S. Pat. Nos. 3,913,668 and 4,046,407 which relate to rigid satellite pipes.
However, experience has shown that the bent portions of the satellite pipes then withstand the highest stresses, and that these stresses become prohibitive for high inclinations of the riser column. It has also been ascertained that upon bending of this column, the helical portions of the satellite pipes are deformed until they come into abutment against each other.
It is also known from U.S. Pat. Nos. 3,426,843 and 4,031,919 to substitute for the rigid satellite pipes, helical hoses arranged substantially helically around the flexible assembly at the lower part of the riser column and which are directly connected to the rigid satellite pipes arranged substantially parallel to the axis of the central pipe. Tests have shown that high mechanical stresses were developed at the level of these connections and that for high bending angles of the riser, the corresponding deformations in the profiles of the flexible hoses lead to entangling of these flexible pipes.
Most of the presently available flexible riser columns suitable for average depths (150 to 300 meters) are not capable, in the absence of special operating steps, of withstanding the extreme operating conditions which may be encountered during a working life of about 20 years. Such special operations may in particular consist of disconnecting or even of lifting the riser assembly.
Moreover, in the prior art riser columns, the angular rotation induced at the foot of the column by these extreme operations exceeds the capacity of the flexible couplings (flex-joints) and/or of the satellite pipes.
As a matter of fact, different recent studies have shown that the angular movement induced at the articulated foot of a production riser column under extreme conditions may exceed 10.degree., this value representing the limit of the angular movements of ball and socket joints and flexible couplings (flex-joint) which are available at present. Other studies have shown that the hydrocarbon deposits whose exploitation from a floating support can be expected in a near future will require the use of a riser column equipped with about twenty satellite pipes used as production, fluid injection and servicing pipes.